The structure of the Reiser file system
Example:
The following example is an entire directory item, that belongs to the stat item example from the previous section:
00000000 01 00 00 00 02 00 00 00 0e 00 00 00 48 00 04 00 ............H...
00000010 02 00 00 00 01 00 00 00 02 00 00 00 40 00 04 00 ............@...
00000020 00 6d 6f 73 0e 00 00 00 60 00 00 00 30 00 04 00 .mos....`...0...
00000030 76 69 2e 72 65 63 6f 76 65 72 00 00 00 00 00 00 vi.recover......
00000040 2e 2e 00 00 00 00 00 00 2e 00 00 00 00 00 00 00 ................
Header 0: {hash 0, gen. 1, 2, 14, byte 0x48, 4 (bit 2 set: visible)}
Header 1: {hash 0, gen. 2, 1, 2, byte 0x40, 4 (bit 2 set: visible)}
Header 2: {hash 15130330, gen. 0, 14, 96, byte 0x30, 4 (bit 2 set: visible)}
Name 2: "vi.recover"
Name 1: ".."
Name 0: "."
As one can see, the directory referenced by key {2, 14, 0, 0} consists of 3 entries, which in return have the following keys (all these keys will lead to the stat item for the directory first):
| . |
{2, 14, 0, 0} |
| .. |
{1, 2, 0, 0} |
| vi.recover |
{14, 96, 0, 0} |
Direct Items Direct items contain the entire file body of small files or the tail of a file. For small files, all the necessary other information can be found in the item header and the corresponding stat item for the file. For the tail of a file, the key for the direct item is the last one for the file.
Indirect Items In direct items contain pointers to unformatted blocks that belong to a file. Each pointer is 4 bytes long and contains the block number of the unformatted block. An indirect item that takes up an entire leaf node can at most contain (blocksize-48) / 4 pointers (the 48 bytes are for the block and item headers). In a partition with 4096 bytes block size, a single indirect item can at most reference 4145152 bytes (4048 KB: 1012 pointers to 4K blocks). Larger files are composed of multiple indirect items, using the offset value in the key, plus a possible tail.
The Journal The journal in reiserfs is a continuous set of disk blocks and it describes transactions made to the file system. Each time the file system is modified in any way, instead of performing the changes directly in the file system, the transactions that belong together (those that need to be atomic so that the file system is in a consistent state) are written into the journal first. At a later point the transactions in the journal will be flushed and, if everything was successful, marked as such.
The journal is of fixed size in the file system. In the 2.4.x Linux implementation the journal size is fixed at 8192 blocks plus one block for the journal header. The journal itself consists of variable-length transactions and a journal header. The journal starts with the list of transactions and the journal header is at the end of the journal. A transaction spans at least three disk blocks and the journal header is exactly one block. The journal is a circular buffer, meaning that once the last block of the journal is reached, it wraps around and uses the first block again.
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